This invention relates to a method of using an aluminum alloy as a substrate for magnetic discs, and more particularly to a method of using an aluminum alloy as a substrate of this kind, which enables magnetic recording on the magnetic disc with enhanced density. This invention also provides an improved aluminum substrate for magnetic discs. This invention further provides an improved magnetic disc and a method of making said magnetic discs.
Conventional magnetic discs include a type comprising a substrate formed of an aluminum alloy, with a film of magnetic recording medium coated over the surface thereof. A typical example of this type substrate is formed of an aluminum alloy according to 5086 of AA (hereinafter called "Aluminum Alloy AA 5086"), which has a chemical composition of 0.20-0.70 % Mn, 3.5-4.5 % Mg, 0.05-0.25 % Cr, and the balance of Al and inevitable impurities.
In recent years there have been keenly desired magnetic discs which satisfy requirements of increased storage capacity, shortened access time, reduced storage cost per bit of information, and reduced weight. An essential requisite for a magnetic disc to meet such requirements is to increase the magnetic recoding density, e.g. the number of bits of information that can be stored in a given area of the magnetic disc (bit density).
To achieve increased magnetic recording density, the magnetic disc should have a smooth surface free of defects such as pinholes, with a reduced and uniform thickness of the maghetic recording medium film.
However, if the thickness of the magnetic recording medium film is reduced, the presence of coarse intermetallic compounds and non-metallic inclusions present in the substrate matrix will cause unevenness of the substrate surface and accordingly unevenness of the medium surface as well as occurrence of defects such as pinholes, resulting in the phenomenon that part of information to be recorded is actually not recorded in the magnetic disc, and other disadvantages. Therefore, the substrate should be free of such coarse intermetallic compounds and non-metallic inclusions. However, according to a substrate formed of Al Alloy AA 5086, although the non-metallic inclusions can be greatly reduced by means of filtration of the molten alloy metal during the casting step for the manufacture of the substrate, coarse intermetallic compounds are still present in the substrate matrix, thereby making it impossible to reduce the thickness of the magnetic recording medium film below a certain value.
As noted above, the surface smoothness of the magnetic disc will be improved in proportion to increase of the surface smoothness of the substrate. To this end, conventionally the substrate surface has been lapped into a mirror finish surface so as to achieve required smoothness. However, Al Alloy AA 5086 does not have satisfactory hardness enough to facilitate the polishing.
Furthermore, although the higher the strength of the substrate, the more easily the magnetic disc can be made thinner, and accordingly more compact in size and reduced in weight, Al Alloy AA 5086 does not have high strength enough to satisfy these requirements.